+8mm B4.1 has around 281mm wheelbase
Stock DEX210 is around 274mm at max....so +8mm would put it at around 282mm
Stock RB5 is around 270mm which was shorter than both the DEX and a standard B4.1.....so with +13mm it would be at around 283mm. I haven't heard of too many people that like the +13 RB5...most seem to like around +10mm. A friend of mine runs a +8 and he loves it.
I definitely think the DEX210 could use a little more length......+6-8mm should be perfect.